Hey Ben,
>> "We are using your solution since two years and the job is done. So, thank you for your product !"
No problem, we'll glad to help.
>> "Can we safely delete file "product.1.0.to.1.2.wyu" even in case of "catch-all updates" "
No, you would need to keep all the files around. In the next version of wyBuild we're making it easier to deliver a single file to offline updates.